1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of performing uplink synchronization in a wireless communication system.
2. Related Art
Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future. Reduction of cost per bit, increase of service availability, flexible use of frequency bands, simple structure and open interface, proper power consumption of a user equipment (UE), and the like are defined as requirements.
In general, there are one or more cells within the coverage of a base station (BS). One cell may include a plurality of UEs. A UE is generally subjected to a random access procedure to access a network. The random access procedure is performed by the UE for the following purposes: (1) initial access; (2) handover; (3) scheduling request; and (4) timing synchronization. However, this is for exemplary purposes only, and thus the content or number of purposes for performing the random access procedure may vary depending on systems.
The random access procedure can be classified into a contention based random access procedure and a non-contention based random access procedure. Major difference between the two random access procedures lies in whether a random access preamble is dedicatedly assigned to one UE. In the non-contention based access procedure, since a UE uses only the random access preamble dedicatedly assigned to the UE, contention (or collision) with another UE does not occur. The contention occurs when two or more UEs attempt the random access procedure by using the same random access preamble through the same resource. In the contention based random access procedure, there is a possibility of contention since a random access preamble used by the UEs is randomly selected.
In an orthogonal frequency division multiplexing (OFDM)-based wireless communication system, timing synchronization between a UE and a BS is important so as to minimize interference between users. The random access procedure is performed for uplink synchronization. While the random access procedure is performed, the UE is time-synchronized according to a time alignment value transmitted from the BS. When uplink synchronization is achieved, the UE runs a time alignment timer. If the time alignment timer is running, it is regarded that the UE and the BS are uplink-synchronized with each other. If the time alignment timer is expired or is not running, it is regarded that the UE and the BS are not synchronized with each other. In this case, uplink transmission cannot be achieved except for transmission of the random access preamble.
A random access failure may occur in the contention based random access procedure since there is always a possibility of contention. Therefore, a method is needed for performing uplink synchronization caused by the random access failure.